Cobalt drill bits are particularly successful in cutting through hard metals such as stainless steel iron heads and are always prefered for use on softer material such as carbon steel iron heads. Cobalt drill bits are not made of pure cobalt, but rather a steel alloy with 5 to 8 percent cobalt. The 5-percent alloy is known as M35 grade, and the 8-percent alloy is M42. The cobalt increases the strength of the steel and makes it more heat-resistant; this is an important factor in drilling hard materials because the friction of metal against metal can produce high temperatures that damage the material or the drill bit.
The R drill bit is perfect for cleaning out titanium and steel wood heads that may have debris or a slight burr inside the hosel.
Opening a .355" taper hosel to remove debris or expanding the bore size the hosel to accept a .370" parallel shaft is easily done by step drilling using the T first, U second and 3/8" drill bit last.